Field of the Present Invention
The present invention relates to a lithographic apparatus with a metrology system for measuring a position of a substrate table.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Within the lithographic apparatus it may be important that the position of a moveable item, e.g., the substrate table may be measured with a high precision by a metrology system with respect to a reference position, e.g., the projection system. For this purpose a metrology frame may be connected to at least a portion of the projection system to provide a reference. A reference grid plate may be mounted to the metrology frame and the grid plate may be used by an encoder mounted to the substrate table to measure the position of the substrate table. The grid plate may be sensitive for vibrations caused by turbulence caused by the moving substrate table.